One type of drill used hitherto has been a chainsaw which is modified so as to remove the cutting blade and chain and attach an auger attachment. Essentially the engine of the chainsaw provides the power and the auger attachment creates the drill. Such modified chainsaws have been known for over 30 years.
More recently, dedicated drills had been produced by a range of manufacturers including Tanaka, Stihl and Echo. Such drills comprise a chuck to hold an auger bit, a gearbox and a small internal combustion engine. One of the handles by which the drill is held by an operator in use, is a substantially vertical handle which is at the rear of the machine. Typically an unbalanced second handle is provided which extends substantially horizontally and to the left (and occasionally selectively either the left or right) of the machine and is located towards the middle of machine. Thus in operation such prior art machines are held by the operator with the operator's left-hand on the second handle with the palm of the left hand horizontal, and with the operator's right hand on the first handle with the palm of the right hand vertical. The first handle generally also contains a throttle control lever or trigger.
Typically the gasoline engines of such prior art drills have an operational speed of approximately 7000-9000 rpm and the gearboxes have gear reduction ratios between approximately 15:1 to 30:1.
In rural fencing, it is common to use hardwood fence posts in which multiple holes for spaced apart multiple wires are drilled using a wood auger having a diameter of approximately 1¼ inch (32 mm) since this is a convenient size to receive a single strand of the barbed wire which is almost universally used in rural fencing. A wood auger of this diameter produces a very strong reactive torque in the drill which tends to move the drill unit in the opposite direction of rotation to that of the drill auger. This reactive torque must be counteracted by the operator who must maintain a strong grip on both handles.
Many fence posts, particularly fence posts used for corners and fence posts used for gates and stockyards, have a large cross-sectional size, typically with a transverse dimension of approximately 10-15 inches (250 mm-375 mm). In order to drill a hole 10-15 inches in length, it is typically necessary to reverse the auger rotation every 2-3 inches (50-75 mm) of drilling (depending on the timber variety) to clear the drilled hole of wood chips, and also prevent the auger from binding up and thereby severely jamming.
Some prior art drill units are provided with a reverse gear. When drilling with these prior art units, in order to clear a drilled hole of woodchips, for example, it is necessary to undertake the following procedure:                1. De-accelerate the throttle so as to reduce the engine speed to idle speed,        2. Remove left hand from left handle whilst balancing the drill with the right hand,        3. Using the left hand, manually engage the reverse gear lever and, if necessary, juggle the gear teeth so that they mesh and thus engage the reverse gear,        4. Return left hand to left handle,        5. Accelerate the engine with the reverse gear engaged so as to extract the auger,        6. Repeat step 1,        7. Repeat step 2,        8. Using the left hand, manually engage the forward gear lever and, if necessary, juggle the gear teeth so that they mesh and thus engage the forward gear,        9. Repeat step 4, and        10. Accelerate the engine with the forward gear engaged so as to rotate and advance the auger.        
This operational procedure is very cumbersome and time-consuming. Furthermore, because the quickly rotating auger can hit a hidden knot, or a similarly hidden split in the wood, the auger is liable to suddenly jam without any warning. Such a jam causes a sudden reactive torque of severe magnitude which often causes the operator to lose his grip with his left hand. The consequence is that the drill unit suddenly spins out of control whilst the operator's right hand is still holding the trigger throttle control at a position near to, or at, its full rpm setting. This can cause injuries to the operator's hand, wrist or fingers, and broken bones are by no means unknown. The fundamental problem is that the suddenness with which a jam occurs does not give the operator sufficient time to release the trigger throttle of the engine, and thereby increase the operator's chance of avoiding injury.